I hope this doesn’t come across as making fun, but it had never occurred to me that GitHub could be anything more than a tool for hosting my source code. So if you had written this same piece about all the good times you’d had in Windows Explorer, I’d be no less confused.
Can I ask what was there that made you visit the site for anything other than reviewing pull requests and issues?
It's probably not GitHub as such, but the associated memories and experiences. You never miss a place, you miss the feeling of happines you had when you were there, or the people you spent time with there.
People get emotional over a car, over a house, over a pet... you could argue for everything it's just a car/house/pet... you can get a new one.
Putting pets in that same category is harsh. Pets are family members, living things that you share a home and good times and experiences with.